The present invention relates to a lock cylinder which can be actuated by a knob on the one side and by a key on the opposite side, the cylinder having a locking-member hub arranged in the center region and rotatably connected to the knob and a coupling piece, arranged centrally to said hub, for transmitting the locking rotation caused by the key to the locking-member hub, and having a cylinder core which has tumblers adapted to be positioned by the key.
Such a lock cylinder is known from Federal Republic of Germany Utility Model 72 47 108, in which both the knob and the coupling member are connected in rotation to the locking-member hub. The coupling member is of approximately T-shape. Its crossbar which is developed as a rotary disk is rotatably mounted in a cavity in the inner end of the cylinder core while the T stem engages in form-locked manner into a recess in the center wall of the hub. A compression spring urges the coupling member in the direction towards the cylinder core. The displaceability of the coupling member is limited by the bottom of the cavity on the end of the cylinder core. The coupling member furthermore has an incision for the entrance of the tip of the key. After tumblers have been positioned, the tip of the key enters into the incision in the coupling member and carries the latter along, together with the locking-member hub and the knob. Upon a locking actuation by means of the knob, the locking-member hub and the coupling member are also carried along, while the cylinder core, which is blocked against turning by the tumblers, remains stationary. A lock cylinder developed in this manner is of complicated construction. Upon actuation by key, the locking force must be applied by the key or the key tip. Furthermore, a lock cylinder developed in this manner, when used in mortise locks, requires at least a 360.degree. rotation of the locking-member hub in order to advance the bolt and then be able to withdraw the key.